When the Muslims arrived at Medina, they used to assemble for the prayer,
and used to guess the time for it. During those days, the practice of Adhan
for the prayers had not been introduced yet. Once they discussed this problem
regarding the call for prayer. Some people suggested the use of a bell
like the Christians, others proposed a trumpet like the horn used by the
Jews, but 'Umar was the first to suggest that a man should call (the people)
for the prayer; so Allah's Apostle ordered Bilal to get up and pronounce
the Adhan for prayers.
(Sahih Bukhari 1.578.
see also
Sahih Bukhari 1.577,
Sahih Bukhari 1.580)

This also indicates that the practice of Adhan is a human invention.
This, however, is contradicted by the hadiths in Sunan Abu Da'ud
concerning its origin:

Narrated Mu'adh ibn Jabal:

...

The narrator, Nasr, mentioned the name of the person who had the dream,
saying: And Abdullah ibn Zayd, a man from the Ansar, came. The same
version reads: And he turned his face towards the qiblah and said: Allah
is most great, Allah is most great; I testify that there is no god but
Allah, I testify that there is no god but Allah; I testify that Muhammad
is the Apostle of Allah, I testify that Muhammad is the Apostle of Allah;
come to prayer (he pronounced it twice), come to salvation (he pronounced
it twice); Allah is Most Great, Allah is most great. He then paused for a
while, and then got up and pronounced in a similar way, except that after
the phrase "Come to salvation" he added. "The time for prayer has come,
the time for prayer has come."
....
(Sunan Abu Da'ud 2.507)

Narrated Abu Umayr ibn Anas:

Abu Umayr reported on the authority of his uncle who was from the Ansar
(the helpers of the Prophet): The Prophet (peace_be_upon_him) was anxious
as to how to gather the people for prayer.

The people told him: Hoist a flag at the time of prayer; when they see it,
they will inform one another. But he (the Prophet) did not like it. Then
someone mentioned to him the horn.

Ziyad said: A horn of the Jews. He (the Prophet) did not like it.
He said: This is the matter of the Jews. Then they mentioned to
him the bell of the Christians. He said: This is the matter of the
Christians. Abdullah ibn Zayd returned anxiously from there because of
the anxiety of the Apostle (peace_be_upon_him). He was then taught the
call to prayer in his dream. Next day he came to the Apostle of Allah
(peace_be_upon_him) and informed him about it.

He said: Apostle of Allah, I was between sleep and wakefulness; all of
a sudden a newcomer came (to me) and taught me the call to prayer. Umar
ibn al-Khattab had also seen it in his dream before, but he kept it
hidden for twenty days.

The Prophet (peace_be_upon_him) said to me (Umar): What did prevent you
from saying it to me?

He said: Abdullah ibn Zayd had already told you about it before me:
hence I was ashamed.

Then the Apostle of Allah (peace_be_upon_him) said: Bilal, stand up,
see what Abdullah ibn Zayd tells you (to do), then do it. Bilal then
called them to prayer.

Abu Bishr reported on the authority of Abu Umayr: The Ansar thought that
if Abdullah ibn Zayd had not been ill on that day, the Apostle of Allah
(peace_be_upon_him) would have made him mu'adhdhin.
(Sunan Abu Da'ud 2.498)

It is interesting to see the differences between the two versions of
the origin of the adhan. The one in Sahih Bukhari says that `Umar was
the one who suggested the adhan, while Sunan Abu Da'ud says that `Umar
was hiding it until Abdullah told Muhammad about it. Nothing that
Sahih Bukhari has a higher degree of authenticity than Sunan Abu
Da'ud (in Islamic theology),
and that Abu Da'ud was born and died (202-275 A.H.)
about later later than al-Bukhari (194-256 A.H.),
there appeared to be some form of reconciliation in
order to portray that the adhan was not a human invention.
Interestingly, even Sunan Abu Da'ud contains no justification that the
dream was a divine direction.

raise voice so that whoever hears will be a witness on Day of
Resurrection,
Sahih Bukhari 1.583

Satan's mischiefs,

Narrated Abu Huraira:

Allah's Apostle said, "When the Adhan is pronounced
Satan takes to his heels and passes wind with noise during his flight in
order not to hear the Adhan. When the Adhan is completed he comes back
and again takes to his heels when the Iqama is pronounced and after its
completion he returns again till he whispers into the heart of the person
(to divert his attention from his prayer) and makes him remember things
which he does not recall to his mind before the prayer and that causes
him to forget how much he has prayed."
(Sahih Bukhari 1.582)